In magnetic resonance phenomenon, a group of nuclei with a magnetic moment are placed in a homogeneous static magnetic field. The nuclei absorb energy from a high frequency (RF) magnetic field rotating at a specific (Larmor) frequency, and emit the absorbed energy as an NMR response after the high frequency (RF) magnetic field is removed. In order to create an image of chemical or physical information in a living body using such a phenomenon, it is necessary to find the spatial position where the magnetic resonance response signal is created, and 2-Dimensional Fourier Transform (2DFT) method are common. By a typical 2DFT method, first, a high frequency (RF) pulse is imposed together with a gradient magnetic field for slice selection. Only nuclei in a specific slice volume is selectively excited, and a transverse relaxation magnetization of nuclei is generated. When a gradient magnetic field for phase encoding is impressed after the high frequency (RF) pulse, although the magnetization rotates at a frequency related to the magnetic field at a particular spatial position, even after the magnetic field is removed, the difference of the frequency is kept as a phase difference. While the gradient magnetic field for frequency encoding is impressed, the received nuclei magnetic resonance (NMR) response signal (echo signal) that is created from the transverse relaxation magnetization and that is picked up by a high frequency (RF) coil is amplified by a first amplifier, and is sampled by an analog-digital converter to output a digital signal. When the magnetization rotates at a frequency according to the magnetic field at the spatial location provided by the gradient magnetic field pulse for frequency encoding, the difference of the frequency is reflected in the frequency of the NMR echo signal. A plurality of echo signals are collected by repeatedly changing the phase encoding in successive such procedures. When Fourier transformation is performed on the collected echo signal f(t) along a frequency encoding axis, a projection F(ωx) along the spatial X-axis is obtained. When Fourier transformation is performed on the projection F(ωx) along a phase encoding axis, a spatial distribution F(ωx, ωy) of the chemical or physical information in the living body is obtained. Thus, the gradient magnetic field pulse is used for encoding spatial position information in the NMR echo signal.
Recently, it is strongly required to improve spatial resolution and to shorten imaging time, and the gradient magnetic field is required to have larger magnetic field intensity, that is, a quicker response in spatial change of the magnetic field and a faster rising edge (slew rate). On the other hand, safety regulations restrict this parameter so that the maximum permissible rate of changing the magnetic field (dB/dt) (for permissible values of peripheral nerve stimulus) decreases. When the magnetic field intensity and the slew rate are high, the rate of changing the magnetic field decreases and the range of linearity between the magnetic field intensity and spatial position becomes narrower.
Therefore, it is desirable to select a proper magnetic gradient coil, each of which has a coil pattern whose intensity characteristic, slew rate characteristic and linearity characteristic are different, within the permitted limitation for the change rate of the magnetic field (dB/dt). For example, U.S. Pat. No. 5,736,858 and U.S. Pat. No. 6,236,208 describe two kinds of coils that are put on two layers and one coil is alternatively selectively used according to the pulse sequence then being used for imaging. However, such a structure where two kinds of coils are put on two different layers can make the diameter of a patient access opening where a patient is inserted smaller, and can thus decrease available access space and can make it difficult to properly access the patient. Moreover, U.S. Pat. No. 5,311,135 describes a terminal that is provided partway along a single gradient coil and the gradient coil part associated therewith is alternatively applied according to the size of an imaged body part.